Although the use of strain gauge technology for converting changes in fluid pressure to related electrical signals is known, there exists a need to provide sensors which are more easily manufactured and which have improved reliability and optimum life expectancy. There is also a need to minimize the cost of producing the sensors so as to make it more economically feasible to use them in given applications and thus increase manufacturing volume with resulting savings in large volume manufacturing techniques.
In issued U.S. Pat. No. 6,763,724, assigned to the assignee of the present invention, a pressure sensor using strain gauge technology is disclosed and claimed comprising a tubular port fitting having a fluid receiving opening at one end and a closed, integrally formed diaphragm at an opposite pedestal end, an angular orientation feature and a locking feature for locking receipt of a support member on the pedestal end. The support member has an aperture flat end wall received over the diaphragm portion, the aperture being in alignment with strain gauge sensor elements glass bonded to the diaphragm portion. Wires are bonded both to the strain gauge sensor elements and to circuit pads on the bonded lobe section of the flexible circuit assembly and encapsulated by silicone gel. The entire teachings and contents of this referenced issued patent are hereby incorporated by reference in its entirety.
In automotive applications, the pressure sensors need to be manufactured in high volume at a low cost and must be very reliable in the harsh environment in safety critical (e.g., braking system) applications. In the assembly of strain gage based pressure and force sensors, the isolation of the sense element diaphragm from mechanical stresses in the electronics board (due to humidity, thermal expansion, and the like) drives the use of a so-called “support ring”. In existing practice, support rings are made of either metal (soldered to the electronics board and butt welded to the sense element port structure) or plastic (glued to the electronics board and snap-fit to the sense element port structure). The support ring and its attachment should provide a stable platform for successful wire bonding between the sense element and the electronics board. Metal support rings also serve as conductive paths between the electronics board decoupling capacitors and the sense element port structure. In existing practice the support ring is an internal component of the sensor assembly and is not subject to handling after a protective sensor housing is installed around it.
It is important for a consistent weld process that the lip of the support ring is uniformly thick, free from burrs, and sits flat against the sense element port. In practice, the lip is difficult to form and defects can result in poor welds due to excessive gaps between support ring and sense element port, burn through of support ring, or a laser welder missing the support ring to port interface location.
In the assembly of pressure sensors a protective gel as known in the art is dispensed over wire bonds, glass, and gages to protect against corrosion (wire bonds) and degradation (glass). Bubbles or voids in the gel, depending on their location, sometimes allow corrosion or degradation, or cause mechanical damage to bond wires or bonds. Because of these problems, it is desirable for the gel to be easily inspectable. Presence of gel (an electrical insulator) on electrical contact spring landing pads can cause unwanted open or intermittent contacts. Typically, wire bond windows in the printed circuit boards (PCBs) through which wires are connected to strain gauges, are generally rectangular in shape. The assembly is dimensioned such that with tolerance stack-up, it is possible for the edge of the glass nearest the outer diameter of the diaphragm to be obscured by the PCB. The close vertical spacing of the PCB to the diaphragm can lead to a lack of gel flow in this area, leaving the glass (on which the strain gauges are mounted) unprotected from degradation (a cause of output signal offset shift). There is a need for a design which easily permits gel flow over the most peripherally located glass. In one conventional solution, gel is prevented from flowing to unwanted locations by a combination of strategic placement of SMT components, and by a plastic gel dam component. However, there is a need to manage gel flow without use of additional components which increase costs, and in some cases require additional circuit board real estate, thereby negatively impacting space constraints.
Conventional pressure sensors used in, for example, automotive brake systems require connections between the pressure sensor and electronic control units (ECU). These connections are often made with springs or spring-loaded pogo pin contacts. Such connections are provided as part of the ECU or as part of the sensor assembly. Conventional pressure sensors typically use relatively expensive pogo pins or springs with one or two diameters. In some applications the springs are not symmetrical and must be loaded by hand in the assembly process. It is critical to provide mechanical guidance to the springs to ensure that they make contact with target pads both inside the sensor and on the ECU. The springs must be prevented from buckling. The springs must move freely along their axes and provide the required contact forces.
Pogo pin solutions are expensive because they require machined housings around the springs and extra sub-assembly operations. Customers do not prefer springs provided as part of the ECU since they require spring procurement, handling, and mechanical guidance. Current solutions with springs provided as part of the sensor assembly require multi-piece sub-assemblies including printed circuit boards (PCB's) and multiple orientations of the parts or other special measures during assembly (to prevent springs from falling out due to gravity). The solution of insert molding springs is difficult because of potentially serious difficulties with methods for properly sealing the mold against the spring, preventing mold flash, and precisely orienting the spring tips towards their target contact pads. Furthermore, the cost of handling springs is not eliminated but merely moved in the supply chain, so no cost benefit accrues.